A device for measuring the flow rate of intake air in an automobile engine, of the type having a resistance element having a resistance temperature coefficient and disposed in the intake air passage and adapted to measure the intake air flow rate using the change in the resistance, is known by, for example, U.S. Pat. No. 3,747,577. This resistance element is heated within the intake air passage of the engine by electric current supplied thereto, and the resistance value thereof is changed in accordance with the flow rate of the intake air in the intake passage.
In automobile engines, the air-fuel ratio is controlled upon detection of the intake air flow rate per one suction stroke of the engine by accumulating the intake air flow rate. It is, therefore, very important to precisely detect the flow rate of the intake air. An inaccurate measurement leads to an incorrect control of the air-fuel ratio.
The size of the hot wire type air flow meter is advantageously small but this type of flow meter has a drawback that it cannot detect the direction of flow of the air. Therefore, it is not possible to measure the correct or true intake air flow rate by a mere accumulation of the output of the hot wire type air flow meter, particularly in the low speed region of operation of a 4-cylinder internal combustion engine.